86 THE AMERICAN JOURNAL OF SOCIOLOGY
the temperature of the earth to a certain degree, the water up to this time having floated in a state of vapor.
4. Action of the water on the crust of the earth ; leveling the prominences, and filling up valleys.
5. Appearance of organisms in the period when the earth is covered with water.
Organisms, in general, are composed largely of water, com- bined with other materials ; they are all semi-fluid aggregates. The differences between the organic and inorganic are in the combinations of the constitutive elements and in the difference of movement. The number of elements to which all can be reduced is seventy-six, forty of which, the most important, suffice by their combinations to form the rocks, water, air, plants, and animals.
At the beginning of organic life the temperature on the earth was very high and very uniform. A marked lowering of the temperature at the two poles occurred, without doubt, at the beginning of the Tertiary period, and up to the time of the appearance of the first glaciers. Life had to recede or adapt itself to the new conditions. During the Quaternary age the temperature fell still lower. Northern and central Asia, Europe, and North America were covered with glaciers, extending from the pole to the Alps in Europe ; likewise, starting from the south pole, there remained only an intermediate zone where organic forms could live ; hence there was a large destruction or trans- formation of species. In the post-glacial period the organic forms returned toward the poles with their new characters and those which they acquired little by little in their journey.
The climate then was not divided into zones as it is now. Following the general law of structure, it was at first homo- geneous ; the vaporous atmosphere which at first enveloped the earth preserved the uniform temperature of a conservatory. Also all the sedentary formations of this period, Silurian and Devonian, contain a special flora and fauna, essentially maritime. Likewise each geological stratum, with its particular flora and fauna, has had its general environment and its corresponding climate. Homogeneity tends always to be more easily differ-
